shell: Disable exposay by default

Add a config file option to enable it, but leave it off by default.  Exposay
still triggers too many lock-ups or stuck grabs and triggers under X even
when the Wayland window doesn't have keyboard focus.
diff --git a/desktop-shell/shell.h b/desktop-shell/shell.h
index 8ef550f..dbb2854 100644
--- a/desktop-shell/shell.h
+++ b/desktop-shell/shell.h
@@ -178,6 +178,7 @@
 	} exposay;
 
 	uint32_t binding_modifier;
+	uint32_t exposay_modifier;
 	enum animation_type win_animation_type;
 	enum animation_type startup_animation_type;
 	enum animation_type focus_animation_type;